Photo-polymerization initiators, which trigger polymerization of an acrylate or the like when irradiated with ultraviolet rays, are widely used. Commercial photo-polymerization initiators are described by, e.g., “Kankou Zairyou List Book,” pp. 55 to 72, 1996, edited by The Technical Association of Photopolymers, Japan, published by Bunshin Shuppan.
Recently, research has been extensively carried out to develop photopolymerization initiators having higher sensitivity than commercial initiators. One of these initiators proposed so far has, in the molecule, a section which absorbs light and another section which generates free radicals. The structure of this initiator is based on a concept that the two sections are allowed to exist in the same molecule, whereby there easily occurs a light-induced electron transfer reaction or a light energy transfer reaction between the two sections, thereby enhancing an efficiency in generating free radicals.
For example, many researchers have discussed photopolymerization initiators comprising an ammonium cation and borate anion, the above ammonium cation having a benzophenone structure, coumarin structure or naphthalene structure as the light-absorbing section [PCT WO97/16406; Journal of American Chemical Society, Vol. 117, pp. 11369 to 11370 (published by American Chemical Society in 1995); Macromolecules, Vol. 29, pp. 8047 to 8052 (published by American Chemical Society in 1996); Macromolecules, Vol. 31, pp. 951 to 954 (published by American Chemical Society in 1998); Macromolecules, Vol. 32, pp. 328 to 330 (published by American Chemical Society in 1999); and Journal of Organic Chemistry, Vol. 64, pp. 458 to 463 (published by American Chemical Society in 1999)].
Moreover, the relationship between electron accepting ability and photolysis efficiency in the cation section of these onium ion type radical polymerization initiators has also been researched, and onium salts having a variety of substituents have been reported. For example, an ammonium borate having a benzothienyl group or benzofuryl group in the molecule is reported in Macromolecules, Vol. 31, pp. 6476 to 6480 (published by American Chemical Society in 1998). This polymerization initiator has absorption at 365 nm, and may act as a polymerization initiator incorporated with a photo-sensitizing section, when irradiated with ultraviolet rays of the same wavelength.
Furthermore, it has been suggested another type of high-sensitivity photopolymerization initiators that has two or more free-radical generating sections in the molecule. Examples of this type include sulfonium borate complexes mentioned in Japanese Patent Laid-open Publication No. 5-218361, Japanese Patent Laid-open Publication No. 5-255347, Japanese Patent Laid-open Publication No. 5-255421, Japanese Patent Laid-open Publication No. 6-157623, Journal of Chemical Society, Chemical Communication, pp. 675 to 676 (published by Royal Society of Chemistry in 1997), Macromolecules, Vol. 31, pp. 6022 to 6029 (published by American Chemical Society in 1998), Journal of Photoscience, Vol. 5, pp. 63 to 67 (published by Korean Society of Photoscience), Macromolecules, Vol. 32, pp. 6545 to 6551 (published by American Chemical Society in 1999), and Journal of Photopolymer Science and Technology, Vol. 12, pp. 115 to 120 (published by Technical association of Photopolymers, Japan in 1999). It has been reported that these initiators show high sensitivity, when irradiated with light, by generating free radicals from both the sulfonium cation and borate anion in the presence of a sensitizer such as Michler's ketone, thioxanthone or ketocoumarin.
Recently, from the viewpoints of productivity improvement and cost reduction, there are generally demanded materials which can be cured at a smaller dose or with a smaller quantity of light energy than a polymerizable composition incorporated with a known conventional polymerization initiator or additive, i.e., more sensitive materials.
Characteristics regarding colors are frequently controlled very strictly in these fields. For example, a color filter resist may suffer deadly deterioration of quality because of an inability to reproduce a target color when colored or discolored by a component other than pigment or dye. More specifically, a blue color filter resist will greatly lose color purity when a component added thereto is discolored yellow, thereby noticeably narrowing a color region reproducible by a liquid crystal display. Demands for a color reproducible region have been increasing particularly in recent years, with the spread of liquid crystal displays.
A variety of irradiation wavelengths have been used in fields in which photo-curing techniques are applied, and most frequently used irradiation wavelengths of ultraviolet rays are 365 and 405 nm. In addition, continuous wavelengths in a range of 340 to 420 nm of a metal halide lamp are also often used. For the polymerizable composition to exhibit high polymerization sensitivity with irradiation by ultraviolet rays of the above wavelength, it is necessary to contain a photopolymerization initiator or sensitizer which exhibits an adequate absorption of the rays. However, many known materials having an adequate absorption of ultraviolet rays at the above wavelength also have an absorption of about 400 nm or a longer wavelength, and so they are colored yellow. On the other hand, materials which are not so colored lack adequate absorption of ultraviolet rays of the above wavelength, and hence they often have insufficient polymerization sensitivity. In order to improve sensitivity by the use of such a material lacking the adequate absorption for the polymerizable composition, it is necessary to increase a quantity of the polymerization initiator or sensitizer to be added. This frequently colors the material in consequence, or leads to the deterioration of film characteristics in the presence of a large quantity of the low molecular weight compound.
Conventional known sulfonium ion type photopolymerization initiators lack absorption of an irradiation wavelength of 365 or 405 nm emitted from a mercury lamp, or a wavelength range of 340 to 420 nm of continuous light emitted from a metal halide lamp. Therefore, these initiators required the complicated task of selecting an optimum sensitizer from a number of sensitizers and determining its optimum content before it is actually used. Moreover, known sensitizers such as Michler's ketone, thioxanthone and ketocoumarin, are frequently colored. They are partly decomposed when irradiated with light, but will not completely be decolored. In consequence, the polymerized composition incorporated with the above sensitizer will invariably be colored, and hence, it is unsuitable for applications which are strictly controlled regarding color-related characteristics typified by color filters described above.
An ammonium borate having a benzophenone structure, coumarin structure or naphthalene structure, or a naphthyl group, benzothienyl group or benzofuryl group in its molecule is not so colored. They have absorption of a wavelength around 365 nm even in the absence of the sensitizer, and can function as the photopolymerization initiator. However, they absorb a wavelength of 365 nm very weakly. Under current situations which demand highly sensitive polymerizable compositions capable of curing materials in a shorter time and with a smaller quantity of irradiation light to reduce cost or improve productivity, their practical sensitivity is insufficient for the increased cost resulting from the incorporation of an expensive borate structure.
A highly sensitive polymerizable composition capable of satisfying all of the requirements described above is not known so far. Therefore, demand is increasing for polymerizable compositions which exhibit, even in the absence of the sensitizer, the high sensitivity for irradiation light of a wavelength in an ultraviolet ray range, particularly irradiation light having a wavelength of 365 or 405 nm emitted from a mercury lamp or an irradiation light range of 340 to 420 nm emitted from a metal halide lamp and which are colored only to a limited extent.